1. Field of the Invention
The present invention relates generally to a wavelength division multiplexing (WDM) optical communication system having optical add drop multiplexing (OADM) capability. More particularly, the invention relates to a wavelength division multiplexing optical communication system featured by its dispersion compensation method.
2. Description of the Related Arts
As a WDM optical communication system, a linear repeater transmission system using an optical amplifier is an effective means for lowering the cost of lines. Such a system has an OADM (Optical Add Drop Multiplexing) capability not only to carry out point to point transmission between terminals, but also to carry out insertion (ADD)/branching (DROP) of some of signals as unchanged light at an intermediary node.
Such an OADM capability is necessary to makeup of a low-cost and flexible optical network. Accordingly, in the WDM linear repeater transmission system, an electrical/optical (E/O) converter is placed at a transmitting-end terminal node, and an optical/electrical (O/E) converter is placed at a receiving-end terminal node. This allows transmission between the receiving-end and transmitting-end terminal nodes to be carried out.
Further, at an intermediary node, an optical signal insert (ADD)/branch (DROP) circuit is placed to add/drop a signal. A passing channel (“through” ch) is allowed to pass through in the form of an intact optical signal, whereas some channels may be inserted (ADD)/branched (DROP)as unchanged optical signals. Such a node is referred to as an OADM node.
A typical WDM optical communication system suffers a waveform distortion due to wave dispersion (time delay per unit duration corresponding to wavelength), which may occur in the optical transmission line. To suppress waveform distortion caused by this waveform dispersion, a dispersion compensator is placed at each Term ILA (Terminal In Line AMP) and OADM node.
Further, for the setting of the amount of compensation in the dispersion compensator, it is necessary to set a target value for the difference (hereinafter referred to as resilient dispersion) between the total amount of dispersion of the transmission line and the total amount of compensation of the dispersion compensator so as to fall within a tolerance range around an optimal value.
On the other hand, in the linear repeater transmission system having an optical amplifier repeater, chirp will occur in the transmission line, due to nonlinear effectiveness (SPM: Self Phase Modulation, where the refractive index of fiber depends on light intensity, or XPM: Cross Phase Modulation, where the refractive index changes as a function of signal intensity of other wavelengths) which may appear in the transmission line. Because of this the target value for residual dispersion will differ in response to the number of spans and span length.
FIGS. 1A and 1B illustrate the above, showing an exemplary configuration of a wavelength division multiplexing (WDM) optical communication system. With the example shown in FIG. 1A, a 3R (retiming, regenerating, reshaping) span between a terminal node #A and a terminal node #B is made up of 15 spans.
OADM nodes #1 and #2 are placed at 5-span intervals, and a signal is inserted (ADD)/branched (DROP) at the OADM nodes #1 and #2.
Accordingly, as shown in FIG. 1B, the formation of path groups {circle around (1)}-{circle around (6)} is possible. The path groups {circle around (4)}, {circle around (5)}, and {circle around (6)} have the span count of 5, and the path groups {circle around (2)} and {circle around (3)} have the span count of 10, and further the path group {circle around (1)} has the longest span count of 15.
In such a configuration, as can be understood from FIG. 2 which shows the range of a transmittable residual dispersion value for each span count, and assuming that the optical fiber is SMF (Single Mode Fiber) and that the distance of 1 span is 100 km, then the optimal resilient dispersion value determined from the transmission characteristics is about −300 [ps/nm], from line A which shows the optimal dispersion value, for the path groups {circle around (4)}, {circle around (5)}, and {circle around (6)} which have the span count of 5; near zero for paths {circle around (2)} and {circle around (3)} which have the span count of 10; and near about +200 [ps/nm] for the path group {circle around (1)} which has the span count of 15.
On the contrary, as a method of setting the residual dispersion value, one method hitherto supposed by inventors of the present invention is shown in FIGS. 3A and 3B. This method, in FIG. 3A, makes use of a transmitting-end dispersion compensator (DCT) 10 which compensates dispersion at the former portion every 1 span, and a line dispersion compensator (DCL) 11 which compensates dispersion at the latter portion every 1 span, to make a residual dispersion value R D which occurred at 1 span equal to zero. In this method, even at each OADM node #1 and #2 the residual dispersion value RD is made equal to zero.
FIGS. 4A and 4B are diagrams showing a different method supposed by the inventors of the present invention. With this method, in FIG. 4A, wavelength dispersion is compensated at the former portion by the transmitting-end dispersion compensator (DCT) 10 every 1 span, and wavelength dispersion is compensated at the latter portion by a line dispersion compensator (DCL) 11 every 1 span. Further, as shown in FIG. 4B at receiving-end dispersion compensators 12, 13, and 14 of the 5th span, the optimal residual dispersion value for the span count 5 path groups {circle around (4)}, {circle around (5)}, and {circle around (6)} is set to be about −300 [ps/nm]. By doing so an optimal residual dispersion value for the path groups {circle around (4)}, {circle around (5)}, and {circle around (6)} can be achieved.
However, in FIGS. 1A and 1B, for the signal of the path group {circle around (1)} which passes through 15 spans at the terminal node #B, the difference from the optimal residual dispersion value becomes large at about 1,100 [ps/nm]. Because of this, the signal of the path group {circle around (1)} cannot be carried to the terminal node #B.
Therefore, with the method shown in FIG. 3, when setting within the tolerance range a residual dispersion value required for the path group {circle around (1)} passing through 15 spans by the dispersion compensator 14 (RD=+200 [ps/nm]), an optimal residual value may become unachievable as shown in FIG. 3B for a signal of the path group {circle around (6)} which is added at the OADM node #2 shown in FIG. 1B, and the transmission may possibly become difficult.
On the other hand, with the method in FIGS. 4A and 4B, for the path group {circle around (1)} passing through 15 spans, the residual dispersion value at the terminal node #B is −900 [ps/nm], and the optimal residual dispersion value cannot be set within the tolerance range shown in FIG. 2. Because of this, transmission is difficult for the signal of the path group {circle around (1)}.